We will examine thyroid hormone effects on cell growth in cultured GC cells, a thyroid hormone-sensitive cell line that produces growth hormone. The G1 period of cells synchronized in the presence of T3 is 12 to 15 hrs, whereas in cells synchronized in the absence of T3, the G1 is 35 hrs. Addition of 0.3 nM T3 to cells synchronized in the absence of T3 decreases the length of the G1 period from more than 35 to 17 hrs. This effect of T3 is restricted to the first 6 to 8 hrs of the G1 period, is stimulated by a 50-fold greater LT4 concentration, and is prevented by cyclohexamide which itself does not appear toxic to the cells. Thus, T3 action on shortening the G1 period and accelerating cell growth is mediated by nuclear T3 receptors and involves the synthesis of new proteins. We will now examine the role of T3 in the induction of specific proteins and phosphoproteins at the G1 commitment point. We will also determine whether the increase in nuclear T3 receptor in S phase is due to a change in nuclear T3 receptor synthesis or degradation rates. Since growth hormone is specifically induced by T3 in the GC cell line, we have determined GH production and secretory rate of the different phases of the cell cycle in relation to changes in concentration of nuclear T3 receptor and chromatin. Growth hormone production and synthesis rate measured by immunoprecipitation increased during S phase in parallel to an increase in nuclear T3 receptor. This is caused by an increase in GH mRNA which is proportional to the increase in growth hormone synthesis. Finally, we measured the synthesis rate of GH mRNA and found it to be decreased during the S phase of the cell cycle. Thus, the increase in mRNA for GH that is observed in the S phase cells results from synthesis at earlier points of the cell cycle. (N)